Buying NOS
I recently bought a fitness tracker band that was new-old stock. The company that makes these has stopped manufacturing this particular unit but as it has a fantastic smart alarm feature I thought I'd get another one.
Upon receiving the tracker I tore open the box and plugged it into a USB port on the computer so it could be charged. It should have only taken an hour, maybe two to be fully charged but after two hours it still showed a charging status on the LED. Oh well, I turned off the computer and hooked the tracker up to a portable power bank for an overnight charge.
The next morning I checked up on the tracker and it still wasn't charged. Taking the tracker off charge and everything just turned off. No operation at all.
I was a little peeved with it and contacted the supplier so I could return it. Because I don't learn very quickly, I got another one. This one had the same problem.
Like nearly every portable device these days, it contains a rechargeable lithium ion battery. While these batteries have a very good state of charge retention the product they are powering may have still draw some current, even during sleep. The battery protection circuits will also draw a small current.
In storage these batteries are best kept in a half charged state. This reduces calendar life reduction due to high voltage and also allows for some capacity to cover that sleep/standby current draw. But eventually, if the product is stored for to long, the battery will end up at a very low voltage.
This is when the protection circuit steps in and forbids charging. A low voltage lithium cell can become dangerous upon charging due to dissolution of the anode material, this can be deposited back on during charge but will not be neatly deposited in the same spots it dissolved from. Dendrites form instead and these can breach the separator, potentially causing micro-shorts. The end result is anything from increased self-discharge to thermal runaway with the possibility of uncontrolled disassembly and fire.
The lesson here is be aware that buying NOS products containing a lithium ion battery may result in disappointment. Also, if storing a device you plan on using again sometime in the future, make sure you give it a small top up charge from time to time.
